The phenomena referred to as "glow discharge" is utilized in a variety of commercial applications, including glass coating, cathode sputtering, reactive ion etching, nitriding and carburizing, to name a few. Basically, a glow discharge occurs when an ionizable gas is introduced into an evacuated chamber between two electrodes of different electrical potential, the potential difference between the electrodes being sufficient to ionize the gas. This potential difference may be anywhere from 200 volts to 800 volts dc.
Once ionized, the positively charged ions are attracted by, and move toward the cathode. A material mounted to the cathode can be coated by impacting ions in this manner. In other glow discharge processes such as sputtering, for example, impacting ions dislodge particles of a target material and cause the dislodged particles to be deposited as a thin film on a substrate to be coated. The substrate is usually mounted to the anode, opposite from the target. Sputter coating of this type is particularly useful in coating a sheet of glass for tinting, or imparting a low emissivity to the glass to block infrared light. Materials such as titanium, zinc, gold, aluminum, or silver may be sputter coated onto glass. A glow discharge apparatus may be planar or cylindrical.
To increase the efficiency of the glow discharge process, a magnet may be located adjacent to the cathode to superimpose a magnetic field over the target and form a denser plasma. A magnetron glow discharge apparatus of this type is disclosed in U.S. Pat. No. 4,422,916, which is expressly incorporated by reference herein in its entirety.
Due to the relatively high d.c. voltage applied between the anode and cathode of a glow discharge apparatus, electrical shorting or arc formation may occur with a relatively high frequency, as often as 2000 times or more per second. While some of these electrical arcs may extinguish due to fluctuation of gas pressure in the chamber, many other electrical arcs will persist until power is disconnected. Current levels during an electrical arc may be as high as 300 amps, depending upon the operating voltage. At these current levels, arcing may damage the power supply, the target material at the cathode or the substrate at the anode. To avoid this potential damage, it is critical that electrical arcs between the electrodes of a glow discharge machine be detected and terminated as quickly as possible.
In some systems electrical arc suppression is accomplished by momentarily disconnecting a rectified a.c. power supply. However, once disconnected, it is possible that power may not be resupplied to the electrodes until another half of an electrical cycle has elapsed. For power supplies operating at relatively low frequencies, this delay presents a problem. For example, a three phase, 6 pulse unit, 60Hz power supply is commonly employed in many glow discharge operations, due to both practical and economical reasons. Once a power supply of this type has been interrupted, it is possible that power will not be resupplied to the electrodes for another 2.66 milliseconds. Additional time is also required for the energy stored in the inductor in the power supply to dissipate. This time is typically about 50 milliseconds. In view of the high frequency of arcing, i.e., as often as every 500 microseconds, this time duration is simply not acceptable.
Even if arcing does not actually occur with such a high degree of frequency, the time delay associated with any arcing at all is considered unacceptable because it reduces the machine's "throughput", i.e., rate of processing, and production efficiency suffers.
While high frequency power supplies, i.e., those on the order of 10KHz, could be employed to solve the above-noted problem by simply reducing the time of an electrical half cycle, high frequency power supplies are generally very expensive, and due to rippling effects, it is difficult to maintain a consistent high power level at the electrodes.
Another approach to arc suppression involves the use of a capacitor connected across the electrodes, with a polarity the same as that of the power supply. When an arc occurs, the capacitor initially dumps its stored charge into the arc, and then extinguishes the arc during recharge. Although this approach does not necessitate temporary disconnection of the power supply, this "dumping" of charge from the capacitor often adversely affects the material mounted at the cathode, an effect sometimes referred to as "splattering."
It is therefore an object of this invention to provide an electrical circuit that quickly and effectively detects and suppresses an electrical arc between two electrodes of a glow discharge apparatus.
It is another object of this invention to provide an electrical circuit that suppresses an arc between two electrodes of a glow discharge apparatus without requiring the time delays normally associated with disconnection of the electrical power supply to the apparatus.
It is still another object of the invention to provide an electrical circuit that effectively suppresses an electrical arc in a glow discharge apparatus supplied by a 60Hz electrical power supply.
It is still another object of this invention to provide a relatively simple and efficient electrical circuit that may be installed between a glow discharge apparatus and a 60Hz power supply in order to provide fast arc detection and suppression.